The present invention relates to a melt-extruded, oriented, self-supporting polyester film coated on one or both surfaces with an adhesion-promoting coating.
Oriented polyester film, especially biaxially oriented film of polyethylene terephthalate (PET), is widely used as a base or carrier material for drafting film, photographic and reprographic film, as well as for packaging and labeling purposes.
PET film is hydrophobic and does not accept coating easily. In most cases where PET film is to serve as a base or carrier film for further coatings, it must first be coated with an adhesion-promoting coating on one or both sides. The adhesion-promoting coating adheres to the film and at the same time is capable of accepting further coatings. U.S. Pat. Nos. 2,627,088 and 2,698,240 describe an adhesion-promoting coating for PET film which is comprised of a terpolymer of vinylidene chloride, acrylic ester and itaconic acid. This adhesion-promoting coating is said to exhibit excellent adhesion to the polyester surface and also to subsequently added water- or alcohol-based photographic gelatin layers.
U.S. Pat. No. 3,674,531 describes an adhesion-promoting coating for PET film which contains copolymers of a vinyl halogen ester, for example, vinyl chloracetate, which can be copolymerized with many other monomers, such as acrylic and methylacrylic acids, -esters and -amides, olefins and vinyl alcohol. Such copolymers can also be cross-linked by the addition of melamine or urea-formaldehyde resin condensates. The adhesion-promoting coated PET film exhibits an improved adhesion with respect to many coatings, including reprographic coatings.
Some of these and other adhesion-promoting coatings for polyester film can, certainly, effectively improve the adhesiveness of PET film. The film manufacturer is also concerned that film waste created during the production process be recoverable. Film waste is normally reduced to small pieces, melted, worked into pellets, remixed with original polyester raw materials, melted again and re-added to the film extruder. During the processing of recovered PET film, temperatures can be between 270 degrees Celsius and 310 degrees C. Many of the above-mentioned adhesion-promoting coatings are not stable at these temperatures. Oriented PET film containing considerable amounts of such recovered adhesion-promoting coatings tends to exhibit an undesirable yellow or black color. This is especially true if the film contains significant amounts of coated film waste which has been reclaimed by repeated passes through the extruder. This is the case with the adhesion-promoting coatings for PET film which contain vinylidene chloride described in U.S. Pat. Nos. 2,627,088 and 2,698,240, as well as with the adhesion-promoting coatings based on copolymers containing vinyl chloracetate according to U.S. Pat. No. 3,674,538. It has been found that the coloration and degradation of these adhesion-promoting layers during the reclamation process can most likely be attributed to the dechlorination or dehydrochlorination and to the degradation products arising therefrom.
U.S. Pat. No. 3,819,773 describes adhesion-promoting thermoset acrylic or methacrylic coatings which can be applied to the PET film from an aqueous medium. Such an adhesion-promoting coating improves the adhesion of reprographic and drafting coatings based on organic solvents. This patent also describes the recovery of an adhesion-coated film, with the therein described thermoset acrylic coatings, by mixing it with at least 50% by weight original polyester raw materials and resupplying the film extruder with this mixture. Although the patent mentions a reduction of discoloration and degradation problems compared with chlorine-containing adhesion-promoting coatings, the acrylic coatings according to the patent, which have been cross-linked with the therein described resin-shaped cross-link-agents, still cause an undesirable yellow coloration of a finished film containing such an adhesion-promoting coated film as reclaim material, especially when compared to a film comprised solely of original polyester raw material.
The object of the present invention is an adhesion-promoting coating for polyester film which, in addition to adhesion-promoting coatings comparable to those known in the state of the art, does not lead to substantial yellowing of the finished film if a film coated with adhesion-promoting coating is added to the production process as reclaim material, especially in comparison to a film consisting solely of virgin polyester raw material.